1. Field of the Invention
This invention generally relates to a transmissive, rear-illuminated twisted-nematic (TN) color liquid crystal display (LCD) employing a special front fiber-optic faceplate or optical equivalent, dual negative retarders, and a light shaping element, such as a brightness enhancing film (BEF), on an illumination source that increase the effective viewing angle between the display and a viewer while minimizing undesirable variations in display chromaticity, luminance, and contrast ratio. In particular, the front fiber-optic faceplate or optical equivalent works in conjunction with the dual negative retarders and the light shaping element to provide an improved contrast ratio with a nearly ideal symmetric viewing cone free from grey scale inversions.
2. Description of Related Art
A conventional, transmissive direct-view color LCD is composed of a source of illumination and a multitude of layered optical elements which each modify the spectral composition of light originating from the source. Moreover, some of these elements, such as polarizers, retardation films and the liquid crystal (LC) layer itself, are optically anisotropic and birefringent layers which produce complex spectral modifications that vary as a function of the material parameters and construction of the LC cell, display voltage (i.e., luminance or gray level), and the direction of light propagation. The predominant LC cell configuration for high-performance color LCDs is the twisted nematic(TN) cell. In the TN cell, incoming light is initially linearly polarized by an entrance polarizer and then the axis of polarization is optically rotated by the LC layer. The rotation of the axis of polarization is mediated by the bifringence birefringence and thickness of the LC layer. The typical twist or rotation angle used for most TN LCDs is 90.degree., although other twist angles may be used to achieve certain desired optical characteristics. After optical rotation by the LC layer, the polarization state of light exiting the LC layer is analyzed by the exit polarizer or "analyzer." Two principle configurations of TN cell entrance and exit polarizers are used, LCDs that utilize crossed polarizers are often called normally-white (NW) mode LCDs while those consisting of parallel polarizers are typically called normally-black (NB) mode LCDs. For both voltage-controlled gray scale and off-axis viewing, the light path through the LC layer encounters a different optical birefringence than in the fully voltage-saturated, on-axis situation. This is due to the fact that the angles at which the light path intercepts the anisotropic LC molecules vary as a function of both LC cell voltage and viewing angle. This in turn results in different degrees of stimulation of the ordinary and extraordinary modes of the LC causing varying degrees of phase difference between the two polarization components, different polarization states at exit from the LC cell, and resulting variations in light transmission through the exit polarizer. In addition, phase differences between polarization components and resulting variations in light transmission are wavelength dependent, thereby resulting in chromaticity differences as well as intensity or luminance differences. Off-axis viewing adds additional complications due to path length differences through all of the material layers comprising the LCD as well as angle-related reflection and polarization effects at all of the different optical boundaries.
As such, LCDs, and in particular TN color LCDs, exhibit undesirable variations in luminance, contrast ratio and chromaticity as a function of the viewing angle between the display and an observer. Moreover, since both off-axis viewing and voltage-controlled gray scale result in variations in display luminance, contrast ratio and chromaticity, the combination of these two factors further accentuates the viewing cone anisotropies evident in direct-view TN LCDs. In some instances, i.e., at particular combinations of viewing-angle and voltage-controlled gray level, the contrast ratio of the display may actually reverse and the desired color may shift to a complementary hue. Obviously, these anisotropies in display visual characteristics greatly limit the useful angular viewing cone of the display, especially for direct-view TN color LCDs employing voltage-modulated gray level control. Thus, while this LCD configuration has many desirable operating characteristics and is therefore commonplace for high-performance direct-view color LCDs (often employing an active-matrix addressing substrate to facilitate high-resolution/high-contrast operation), viewing angle limitations severely compromise the ultimate imaging performance achievable with this display device.
A number of potential solutions to ameliorate viewing angle problems in direct-view LCDs have been proposed; including the use of a diffusing optical layer at the output of the LCD, three-dimensional retardation films or optical compensators e.g., Ong, H. L. (1993). Negative-Birefringence Film-Compensated Multi-Domain TNLCDs with Improved Symmetrical Optical Performance. SID Digest of Technical Papers, 658-661!, and so-called multi-domain pixel structures Tanuma, S. (1988). Japan Patent No. 63-106624; Otani, A. (1989). Japan Patent No. 0188520!. The use of a diffusing optical layer (e.g., ground-glass scattering screen) at the output of the LCD would result in a de-coupling of the LCD from the viewing direction; however, such a diffusing element would scatter light from both directions and would severely degrade LCD image contrast under incident ambient illumination, which is typical for most office and outdoor environments.
Retardation films or optical compensators can produce some limited but useful improvements in LCD viewing angle; however, the phase retardation imparted to the light propagating through the film is highly wavelength sensitive and can thus only compensate for a limited portion of the visible spectrum. Moreover, retardation films can not eliminate LCD grey scale inversion problems. These shortcomings limit the effectiveness of retardation films or optical compensators when used for improving the viewing angle of full-color displays; however, they are a simple and cost effective means of achieving some viewing angle performance improvement.
Retardation films have been discussed in U.S. Pat. No. 5,227,903 to Miazawa et al. titled "Liquid Crystal Display Device with at Least One Biaxial Retardation Film Having N.sub.x &gt;N.sub.z &gt;N.sub.y ", U.S. Pat. No. 5,440,413 to Kikuchi et al. titled "Liquid Crystal Display Device with 2 Adjacent Biaxial Retardation Plates Having N.sub.y &lt;N.sub.z &lt;N.sub.x " and U.S. Pat. No. 5,528,400 to Arakawa titled "Liquid Crystal Display Device Having Negative Uniaxial Anisotropic Film with Inclined Optical Axis and Protective Films".
Arakawa discloses a liquid crystal device composed of a TN liquid crystal cell with a pair of polarizing plates arranged on both sides of the cell. Each plate comprises a polarizer and a pair of protective films, one on each side of the polarizer. Additionally, there are optical compensation sheets between the cell and the polarizing plates. The optical compensation sheets may be located either on one or both sides of the TN liquid crystal cell. The properties of the optical compensation sheets are such that they must be grouped in pairs such that one sheet has an optic axis in the direction of the normal of the film while the other sheet has an optic axis in a direction that is inclined from 5 to 50 degrees from the normal to the film. Both films have an optically negative uniaxial property. The films may be arranged such that light passes through either one of them first. Further the sum of the Re values of the films having an optic axis in a direction of the normal with the Re values of the protective coatings between the polarizers must be in the range of 100 to 400 nm where Re is defined as {(nx+ny)/2-nz}xd. This liquid crystal display device shows improvement in viewing angle and grey scale inversion.
Kikuchi et al discloses a TN liquid crystal cell with a polarizer and analyzer arranged respectively on the incident and exit sides of the cell with at least one biaxial retardation plate arranged between the polarizer and the analyzer. The biaxial retardation plate must have its indices of refraction in the x, y, and z directions described by ny&lt;nz&lt;nx and the product of its refractive index anisotropy with its thickness (.DELTA.nd) must fall within the range of 300 to 400 nm. In this device grey scale inversion is improved and changes in color along the horizontal axis are improved.
Miyazawa discloses a super-twisted nematic (STN) liquid crystal cell with a polarizer and analyzer arranged respectively on the incident and exit sides of the cell and two biaxial retardation plates arranged between a substrate of the TN liquid crystal cell and the analyzer. The biaxial retardation plate nearest the TN liquid crystal cell is arranged such that its phase delay axis is inclined at approximately 40 degrees with respect to the incident-side aligning treatment direction. The biaxial retardation plate nearest the analyzer is arranged such that its phase delay axis is inclined at approximately 5 degrees with respect to the incident-side aligning treatment direction. Both biaxial retardation plates have refractive indices nx, ny, and nz described by the equation ny&lt;nz.ltoreq.ny+0.3(nx-ny). An alternative embodiment has two retardation plates arranged to sandwich the pair of substrates in the TN liquid crystal. When the retardation plates are arranged in this configuration each retardation plate will have refractive indices nx, ny, and nz described by the equation ny+0.3(nx-ny).ltoreq.nz.ltoreq.ny+0.7(nx-ny). The product of the refractive index anisotropy of the STN liquid crystal material and the thickness of the STN liquid crystal layer preferably falls within the range of 800 to 910 nm, while the phase retardation of the biaxial retardation plates ranges from 340 to 450 nm.
Finally, relatively recent developments in multi-domain pixel structures, which are optically self-compensating alignments within the LC cell, can prove to be highly effective at improving the viewing angle of direct-view color LCDs. Multi-domain alignment can provide a symmetric viewing angle and eliminate grey scale inversions if one pixel is divided into four domains; however, the processing required to achieve four separate domains is extensive. In addition, such multi-domain alignments can be difficult to establish precisely and also significantly complicate the manufacture of the LC cell leading to substantially increased costs. A two-domain alignment method is less costly and complicated to manufacture and can improve either the horizontal or vertical viewing angle, depending on the orientation of the domains but can not eliminate grey scale inversions. In addition, there is a tendency for the domain or alignment boundaries to appear as visible borders, patterns and striations in the display, thereby degrading the image quality of the LCD. The inherent manufacturing and cost problems have prevented the wide implementation of multi-domain alignment into liquid crystal display products.
Typical color LCD displays use a patterned, mosaic of color selection filters created within the LC cell itself and registered with the two-dimensional pixel matrix. In addition, a subtractive or stacked color LCD configuration can be created with three sequentially ordered and spectrally-selective LC cells which each subtract or remove an orthogonal component of the visible spectrum. Examples of different configurations of subtractive or stacked color LCDs can be found in U.S. Pat. No. 5,032,007 to Silverstein et. al., U.S. Pat. No. 4,917,465 to Conner et. al., and U.S. Pat. No. 4,416,514 to Plummer. While successful as a full-color LCD light valve for projection displays in which the light rays passing through the stack of subtractive cells are collimated or at least telecentric, the subtractive or stacked LCD arrangement is not desirable for use with a backlit, direct-view LCD due to viewing-angle problems arising from the parallax produced by the relatively thick stack of spectrally-selective cells. For these reasons, spatial-additive color synthesis via a planar mosaic of color selection filters is the preferred approach to achieving full color in direct-view LCDs. Examples of mosaic color filters are shown in U.S. Reissue No. 33,882 to Morozumi, U.S. Pat. No. 4,987,043 to Roosen et al. and U.S. Pat. No. 5,066,512 to Goldowsky et al.
Conventional processing or creation of the patterned mosaic of color selection filters within the LC cell is costly, inefficient and severely limited by material compatibilities with the LC fluid. These filters are placed within the LC cell, which typically has a cell gap width on the order of 3 to 7 microns, in order to reduce viewing parallax in displays with small pixel dimensions. Placing the color selection filters outside of the LC cell would require that the filters be displaced from the pixel-forming apertures within the LC cell a minimum distance equal to the thickness of the LC cell glass, which is typically on the order of approximately 1100 microns. This would result in very significant viewing parallax between a pixel aperture and the associated color selection filter, such that at off-axis viewing angles light rays from an addressed pixel could easily go through the incorrect color selection filter (e.g., light rays from an addressed RED pixel aperture actually going through a GREEN color selection filter).
As such, there exists a need for improved color filter processing and placement; allowing easier processing, the use of more efficient filter materials, and increased color image quality over a larger viewing angle range. If an optical means could be developed to control or constrain the angles at which light propagated through the layers of a direct-view LCD until the final optical interface where the light rays may be expanded to provide a wide viewing angle (thereby effectively decoupling the LCD from the viewing orientation), then absorptive color selection filters could be placed outside the LC cell or highly-efficient, interference-type color selection filters could be employed. In either case, this would enable the color filters to be located on a different optical layer than the LC cell, processed using a wider range of more efficient color filter materials and processing stages, and should result in improved manufacturing yields, reduced production costs, and significantly improved LCD color performance and luminous efficiency.
Fiber-optic faceplates (FOFPs) have been used for contrast enhancement on special-purpose Cathode Ray Tube (CRT) displays, as light-collection elements on the front surface of reflective monochromatic LCDs to enhance the reflected luminance of the display, as light channeling elements for coupling patterned color phosphor mosaics to their respective pixel apertures in rear-illuminated color LCDs, and as image relay elements for coupling the output of image generation devices to photo-recording surfaces for hard-copy applications. Several patents relate to FOFPs. These include U.S. Pat. No. 4,344,668 to Gunther et al.; U.S. Pat. No. 4,349,817 to Hoffman et al.; U.S. Pat. No. 4,558,255 to Genovese et al.; U.S. Pat. No. 4,752,806 to Haas et. al.; U.S. Pat. No. 4,799,050 to Prince et al.; U.S. Pat. Nos. 5,035,490 and 5,181,130, both to Hubby, Jr; U.S. Pat. No. 5,050,965 to Conner et al.; U.S. Pat. No. 5,053,765 to Sonehara et al.; U.S. Pat. No. 5,113,285 to Franklin et al.; and U.S. Pat. No. 5,131,065 to Briggs et al.
Haas et. al. uses a FOFP to channel light emerging from an LC layer to a lens array and then to a photoreceptor. Genovese et. al. use a FOFP to channel light emitted by a vacuum fluorescent device to expose a photosensitive member for a printing device. These applications do not relate to direct-view display devices.
Briggs et. al. uses a front FOFP to channel light emerging from an emissive phosphor layer to a viewer in order to create a high luminance and high contrast thin-film electro-luminescent display. Prince et. al. employ a FOFP as a light channeling element for coupling the emissions of a patterned color phosphor mosaic excited by an ultra-violet source to their respective pixel apertures in a rear-illuminated color LCD. These patents relate to the channeling of phosphor emissions in direct-view display devices and are not directly concerned with the improvement of off-axis viewing.
Hubby, Gunther et. al. and Hoffman et. al. all relate to reflective LCD devices that use a FOFP to collect incident light from a wider acceptance angle for the purposes of enhancing the reflected luminance and contrast of the display. This approach does not address the generation of color in an LCD and, in fact, is not applicable to a color LCD because there is not sufficient reflected luminance in such a LCD device to enable color separation and filtering and still provide enough output luminance for comfortable viewing. Moreover, this approach is not concerned with enhancing off-axis viewing performance.
Conner et. al. relates to a STN LCD that requires a collimated light source and uses sequentially-stacked subtractive color LC cells. The primary approach is intended for projection display applications. When applied to the direct-view situation, the display output requires decollimation or diffusion. This results in degraded image contrast and color desaturation under ambient illumination. This approach does not directly address high-performance, direct-view, transmissive TN color LCDs.
None of these references appreciate the problems overcome by the present invention.
U.S. Pat. No. 5,442,467, filed on Mar. 21, 1994 by Silverstein et al., the subject matter of which is incorporated herein by reference, discloses a direct-view rear-illuminated LCD device, comprising: a backlight source; a rear diffuser layer; a rear polarizer; a LC cell including a rear glass layer with addressing elements and indium tin oxide (ITO) transparent pixel electrodes, a LC layer having a top and bottom surface, and a front FOFP as a front containing element of the LC cell and being located directly in contact with the top surface of the liquid crystal layer; a mosaic array of color absorption filters either deposited on the front face of the FOFP or located on a separate but adjacent substrate; and a front polarizer or analyzer. The front FOFP provides for a relatively narrow light acceptance solid angle (.theta..sub.Max IN) at a rear face adjacent the LC layer and a relatively wide light exit or output solid angle (.theta..sub.Max OUT) at a front face opposite the rear face.
For purposes of the present invention, it should be understood that the term fiber-optic faceplate or FOFP is interpreted in its broadest sense as any material which embodies the essential optical properties of a FOFP. Thus, the functioning of the present invention is not dependent upon the use of a fused plate of optical fibers but rather on any material layer, including a fused plate of optical fibers, which is capable of total internal reflection, controllable NA at input and output surfaces, rotational azimuthal averaging and translation of the object plane from the back surface of the layer to the front surface of the layer. It should be apparent to those skilled in the art that these essential optical properties could be imparted to a range of materials, thus producing FOFP optical equivalents. These could include micro-machined or preformed glass or plastic substrates with a plurality of optical features, a variety of polymer networks containing a duality of materials with differing refractive indices or birefringence produced by physical alignment or stress, or any other approach able to result in a substrate containing a plurality of cylindrical features whose boundaries are defined by a discontinuity of refractive indices wherein the index of refraction within the cylindrical features is greater than the index of refraction at the boundaries and external to the cylindrical features.